By with ZHANG Wen 

Figure 1. Measured Trec of superconducting NbN HEB devices as a function of Tbath/Tc. The data are normalized to their respective values at 4.2 K. For comparison, results in [12] and [13] are also plotted in figure . Solid and broken lines are obtained from equation (1) with n = 4 and 3, respectively. The inset shows the simulated mixer noise temperature of device No.1 as a function of Tbath and measured Trec with the setup optical losses corrected.

Researchers led by Prof. Sheng-Cai Shi, Purple Mountain Observatory, Chinese Academy of Sciences, study the temperature dependence of the receiver noise temperature and IF bandwidth of superconducting hot electron bolometer mixer. Superconducting hot electron bolometer mixers have been thoroughly studied at liquid helium bath temperature. Due to the strong atmospheric absorption of the THz radiation, superconducting HEB mixers are useful particularly in air- and space-borne THz heterodyne projects, which are of limited cooling capacity. It is therefore of particular interest to investigate how they perform beyond LHe temperature. In addition, studying their temperature dependence may help further understand the energy relaxation in superconducting HEB microbridges. 

We fabricated three superconducting hot electron bolometer mixers with different processes and measured their receiver noise temperature and IF noise bandwidth. Three superconducting NbN HEB devices of different transition temperatures are measured at 0.85 THz and 1.4 THz at different bath temperatures between 4 K and 9 K. Measurement results demonstrate that the receiver noise temperature of superconducting NbN HEB devices is nearly constant for T_bath/T_c, less than 0.8, which is consistent with the simulation based on a distributed hot-spot model. In addition, the IF noise bandwidth appears independent of T_bath/T_c, indicating the dominance of phonon cooling in the investigated HEB devices. This study has demonstrated that superconducting NbN HEB mixers still have high sensitivity and reasonably wide IF bandwidth at temperatures approaching the device transition temperature. The work by W. Zhang, W. Miao, J.Q. Zhong, S.C. Shi (corresponding author), D. J. Hayton, N. Vercruyssen, J. R. Gao, and G. N. Goltsman, accepted by Superconductor Science and technology, has been published online ( http://iopscience.iop.org/0953-2048/27/8/085013/.